Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins (OTO), particularly the conversion of methanol to olefins (MTO), in a hydrocarbon conversion apparatus generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the OTO and MTO processes incorporate a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst and forms a regenerated catalyst. The regenerated catalyst then may be reused to catalyze the conversion of methanol to olefins.
In conventional regeneration vessels, coked catalyst is directed from a reactor to a catalyst regenerator. In a catalyst regenerator, a regeneration medium, usually oxygen, enters the regenerator, and coke is removed from the coked catalyst by combustion with the regeneration medium to form regenerated catalyst and gaseous byproducts. The bulk of the regenerated catalyst from the regenerator is returned to the reactor. The gaseous byproducts are forced out an exhaust outlet oriented in the upper section of the catalyst regenerator. Undesirably, a significant amount of entrained catalyst from the regenerator is forced out the exhaust outlet with the gaseous byproducts. This loss of entrained catalyst from a catalyst regenerator results in increased costs, particularly on an industrial scale.
Many conventional regenerators operate in a fast-fluidized catalyst flow scheme. For example, Published PCT application WO 02/08359 A1 discloses a regenerator vessel, which in operation includes a fluidized bed zone of catalyst at its lower end. In the fluidized bed zone, a vertically extending partition, which partition is provided with one or more openings, is present dividing the fluidized bed zone in a dense phase fluidized bed zone and a fast-fluidised bed zone, the dense phase fluidized bed zone provided with the means to supply catalyst and the fast-fluidised bed zone provided with the means to supply an oxygenate gas at its lower end. The relatively high velocities in fast-fluidized regeneration schemes impart increased momentum to the catalyst particles contained in the system. As a result, entrained catalyst loss and catalyst attrition are even greater problems in fast-fluidized regeneration systems.
For a variety of reasons, molecular sieve catalyst compositions implemented in OTO and MTO reaction processes are particularly costly to manufacture and can be more than an order of magnitude higher in cost to manufacture than catalysts used in FCC systems. This increased cost is due to the increased costs associated with the individual components, e.g., molecular sieves, that are used to form the catalyst. For example, specialized templates typically are used to form molecular sieves used in OTO and MTO reaction systems. Additionally, the yield of acceptable catalyst formed in a catalyst formulation batch is lower for OTO catalyst than in FCC catalyst manufacture. As a result, entrained catalyst loss from catalyst regeneration is particularly a problem in OTO and MTO regeneration systems. It is therefore desirable to reduce the amount of entrained catalyst loss in an OTO or an MTO regeneration system.